The present invention relates to a laser beam printer and more particularly to a laser beam printer capable of changing the printing speed and quality.
Various types of printers have been developed along with the propagation of work stations and word processors. Among such various types of printers, laser beam printers have been attracting much attention as printers capable of quietly printing at a high speed on ordinary paper.
FIG. 1 illustrates a schematic construction of a laser beam printer which has been used heretofore. A main unit 12 of this laser beam printer 11 is provided with a laser scanning unit 13 installed on its upper area, and this laser scanning unit 13 is constructed in such a way as to irradiate a laser beam as modulated in accordance with image signals onto a photoreceptor drum 15. Such component parts as a charge corotron 16, a developing unit 17, a transfer corotron 18, and a cleaning device 19 are arranged on the circumference of the photoreceptor drum 15.
In this construction, the charge corotron 16 performs the function to charge the photoreceptor drum 15 uniformly. The photoreceptor drum 15 thus charged loses its electric charge selectively in accordance with a scanning operation with a laser beam and forms a latent electrostatic image, Such a latent electrostatic image is developed by the developing unit 17, and a toner image is thereby produced on the surface of the drum. This toner image is transferred onto printing paper 23, which is fed out by semilunar rollers 22 from a cassette tray 21. The printing paper on which the toner image has thus been transferred is then fused by a fusing unit 24 and is thereafter discharged out of the printer. In FIG. 1, a broken line indicates a transport path 26 for the printing paper 23 which is fed out of the cassette tray 21.
Incidentally, a laser beam printer has a fixed printing speed and a fixed scanning line density, which is the density of the scanning lines per unit time, in the same way as in the ordinary electrostatic type copying machine, owing to the necessity of controlling the photoreceptor drum 15 at a constant speed. As regards the scanning line density, 240 dots/inch (dpi) to 300 dpi used to be common among the laser beam printers in the initial period, but laser beam printers capable of attaining 400 dpi to 600 dpi have recently been forming a dominant current. This trend of development is in line with the progressive diffusion of the so-called desk top publication along with the recent ever-increasing acceptance of input/output equipments for processing picturer image information at a significant step forward marked in departure from the former practice of primarily printing text information with such equipments. Also for text information, printers or the like are required to fulfill the needs of smoothing of fonts, and there has arisen a tendency to give preference to outline fonts.
In the current state of affairs marked by progressive improvements achieved on the quality of printed images as mentioned above, the processing of images in a laser beam printer becomes more complicated to that extent and also the number of scanning lines is increased as much. In this situation, it is an actual state at present that the printing speed of laser beam printers has not yet attained any considerable improvement even though various kinds of techniques have thus developed. Therefore, as the amount of work increases in an office equipped with a laser beam printer, it becomes necessary to install an additional laser beam printer, which presents the problem that the need causes a heavier economic burden on the operating expenses.
Also, there has been another problem that a larger capacity has come to be required of the page memory for storing the images for one page along with the improvements on the quality of images to be printed and that this requirement has given considerable influence to the costs of laser beam printers.
Moreover, there has been still another problem that a laser beam printer, which processes picture image information in addition to text information, takes a considerably longer time in its processing of picture image information than in the processing of text information, so that considerable fluctuations occur in the amount of time required of such a printer from the start of the processing of information to the finish of printing, depending on the contents of information to be printed.